1. Field of the Invention
The present invention relates to the technique of removing carbon monoxide contained in a hydrogen-rich reformed gas (an example of “treatment-object gas” as so referred to in the present application) such as obtained e.g. in the reforming process of hydrocarbon fuels including natural gas, naphtha, kerosene, etc, or alcoholic fuels such as methanol.
The technique to which the present application relates is characterized that it can remove carbon monoxide up to a concentration of ten ppm or lower. For this ability, the-technique can be suitably employed in a power generating system using e.g. solid polymer electrolyte fuel cell which operates at a relatively low temperature.
For the purpose of simplifying the description, the following description will be made by taking a reformed gas used in a fuel cell as an example of the treatment-object gas.
2. Description of Related Art
Conventionally, with a fuel reforming apparatus using fossil fuel such as natural gas as raw fuel, a carbon monoxide shift converter is connected to the downstream end of the reformer so as to convert carbon monoxide in the reformed gas into carbon dioxide by the water-gas shift reaction, whereby the carbon monoxide concentration is reduced (removed) to 1% approximately.
On the other hand, with a fuel reforming apparatus using methanol as raw fuel, since this apparatus involves a step of the water-gas shift reaction, the carbon monoxide concentration is reduced (removed) to 1% approximately by appropriately maintaining the operating temperature and the water vapor ratio.
An example of an apparatus to which the reformed gas obtained above is to be fed is a polymer electrolyte fuel cell which is one type of fuel cell.
With this type of fuel cell, since it operates at a low temperature around about 80° C., if the reformed gas, as the fuel gas, contains carbon monoxide even by a trace amount (e.g. greater than several tens of ppm), its electrode catalyst is poisoned by the carbon monoxide, leading to significant deterioration in the cell performance. Therefore, it is necessary to reduce the carbon monoxide concentration in the fed reformed gas to less than several tens of ppm, more preferably to less than 10 ppm. In other words, the carbon monoxide concentration in the hydrogen-rich reformed gas needs to be reduced (removed) by a higher level than the conventional standard level (about 1%).
For the purpose of such relatively high level reduction of carbon monoxide, the method thus far has proposed the following methods.    (a) A CO remover having a metal catalysis is provided on the downstream of the reformer, so that with supply of air or oxygen as an oxidizing agent, carbon monoxide contained in the reformed gas is oxidized to be removed as carbon dioxide.    (b) A “methanator” is provided for causing reaction between hydrogen and carbon monoxide contained in the reformed gas, so that the carbon monoxide is reduced to be removed as methane.
Examples of the method (a) above include the following.    1. “The 2nd FCDIC Fuel Cell Symposium Lecture Proceedings: 235-240 (1995)”. In this, air is mixed with the reformed gas so as to achieve: [O2]/[CO]=3. Then, as this mixture gas is caused to contact Ru catalyst, carbon monoxide in the reformed gas is selectively oxidized and removed.    2. Japanese laid-open patent gazette: No. Hei. 7-296837: “Reformed-Gas Supplying System”. In this, a methanol fuel reforming system includes a methanol retriever disposed at the downstream of a methanol reformer and also includes a carbon-monoxide oxidation reactor (acting as a CO remover) charged with Pt-Rh catalyst disposed at the downstream of the methanol retiever, so as to oxidize and remove the carbon monoxide in a methanol reformed gas.
Examples of the art (b) above include the following.    1. Japanese laid-open patent gazette No. Hei. 6-283189: “Fuel-Cell Power Generating System”. In this, on the downstream of a CO shift converter, there are disposed a CO2 adsorber and methanator having an Ni catalyst, so that some of carbon dioxide contained in the reformed gas is adsorbed and removed at the CO2 adsorber and then carbon monoxide and the remaining carbon dioxide are methanated by the metanator to be removed as methane.
However, the above-described methods respectively have the following problems.    (a) problem with oxidation removal
In order to sufficiently remove carbon monoxide, it is necessary to add oxygen by an amount greater than 6 chemical equivalent. Then, not only the carbon monoxide to be removed, a great amount of hydrogen which can be a useful fuel will be lost by combustion    (b) problem with removal using methanator
With this technique, if the treatment-object gas contains also carbon dioxide as is the case with a reformed gas, methanation of carbon dioxide, in addition to that of carbon monoxide, tends to occur with very high likelihood For this reason, if carbon monoxide is to be removed sufficiently while restricting loss of hydrogen due to methanation of carbon dioxide, it is necessary to first absorb and remove the carbon dioxide also present in the reformed gas, so that the system required for this tends to be complicated.